Systems for the refueling of transportation vehicles powered by cryogenic liquids face a variety of design challenges primarily due to the problems associated with using cryogenic liquids. Insulated containers designed to handle cryogens experience some degree of heat absorption. Tanks containing residual quantities of liquid cryogen, can easily generate boil off vapor during storage or during refueling due to the heat present in the piping or tanks. In some circumstances, the vapor pressure can exceed the maximum allowable operating pressure of the tank design or maximum available transfer pressure available from a dispenser system.
Boil-off vapor from cryogens such as liquid natural gas (LNG), may include gasses such as methane which are condensable back into reusable liquid phase by exposure to the excess cold available in LNG. Other boil-off gasses such as nitrogen, do not condense as easily and may accumulate above the liquid phase level during the refueling cycle. If not vented, increase tank pressure occurs as the liquid phase rises.
The process of refueling containers with LNG has traditionally been accomplished by the use of a two line system where one line conduits vapor away from the fuel tank and a second conduit carries the liquid phase for refueling the tank. The process requires either the use of two lines and two nozzles or a single coaxial line. A coaxial line is both cumbersome and requires a coaxial connection to the vehicle which has high maintenance requirements.
While there has been some work with single hose dispensing systems, the systems do not provide for the depressurization of the fuel tank or the venting of non-condensable gases without the connection of a second smaller cryogenic vent line.
In those systems which require the operator to determine the tank conditions through external mounted gauges, operator error is compounded by frost encrusted line gauges and mechanical damage to the sensors due to excessive vibration and harsh conditions associated with normal vehicle usage. In addition, the operators, for the most part, may have not received extensive training in the handling of cryogenic materials and as such, the connection and disconnection of conduit lines and the dependence on external gauges presents an unnecessary risk to the operator, equipment, and facility.
A variety of methods have been suggested which regulate the vapor phase pressure in tanks by mechanical vent valves which are calibrated to release vapor when a setpoint pressure has been reached. None of these methods utilize a reliable system which reduces the risk of operator error.
In addition to mechanical control mechanisms, a variety of semi-automated systems are known. For example, U.S. Pat. Nos. 5,231,838 and 5,121,609 disclose single line fueling stations for liquid natural gas vehicles where an operator manually controls the venting of vapor phase from a fuel tank to the head gas in a pressure building tank and the delivery of liquid phase gas from the pressure building tank to the vehicle fuel tank by a single line. The systems require a separate liquid nitrogen cooling system to regulate the pressure in the pressure building tank to insure that adequate pressure is available to deliver a desired quantity of liquid natural gas to a vehicle fuel tank.
U.S. Pat. No. 4,080,800 discloses a mechanical control mechanism in which liquid and vapor phases are simultaneously removed from a holding tank through a flow control valve for ultimate utilization as a fuel source.
In U.S. Pat. No. 3,298,186 a refrigeration method is disclosed whereby the ethane content of a refrigerated propane liquid stored in a container is maintained by condensing a portion of the vapors generated from the container and returning the condensate to the container. This method utilizes a co-mingled two phase line for the purpose of reducing the ethane vapor by absorption into the liquid.
One object of the present invention therefore is the controlled removal of vapor and the controlled delivery of single phase liquid cryogen by the use of a single line;
Another object of the present invention is the rapid delivery of liquid cryogen to a variety of transportable fuel tanks having different volumes by a system which is simple to operate;
A further object of the present invention is to provide a refueling system which can deliver desired quantities of cryogenic liquid on demand without requiring the transfer of additional cryogenic liquid for cool down of the lines. A simple system which meets these objects would be a useful advancement in the art of cryogenic fuel delivery systems.
Accordingly, the present invention relates to a system for the automatic refueling of cryogenic containers such as are found in LNG powered vehicles. The system includes as component parts, a primary storage tank and a fuel tank wherein the fuel tank may contain a residual quantity of liquid phase cryogen and also a vapor phase at a positive pressure. The system also includes a dispensing unit which further comprises means for measuring the pressure of the vapor phase, means for calculating the level of the liquid phase in the tank, and means for transporting the vapor phase from the fuel tank to the primary tank and liquid phase from the primary tank to the fuel tank. Also contemplated is a control means for regulating the vapor phase pressure and liquid phase level in the secondary tank.
In operation a single phase transport line is connected to the fuel tank containing residual vapor at a positive pressure and a liquid phase. Values for the fuel tank vapor pressure and liquid phase level are determined at constant time intervals and compared against desired values. If pressure in the tank is above a desired maximum value the pressure is reduced to an acceptable level by transporting the vapor phase to the primary storage tank where the vapor is condensed in the residual liquid minimizing pressure growth in the primary tank. Liquid phase cryogen, is then pumped into the fuel tank and regulated to a desired fill level.
In one embodiment, the operation may be accomplished automatically without requiring any operator involvement thereby reducing the risk of either under filling or over filling the fuel tank due to operator misreading of vehicle based sensor gauges.
Another advantage of the present invention is realized by the elimination of a cool down cycle where LNG is pumped through the lines on pump start up to cool the lines to an acceptable temperature to eliminate flashing and excessive vapor production.